The invention relates to a high-strength hot-rolled steel sheet for use as a constituent material of an automobile wheel and suspension which are worked on by press forming, and in particular, to a high-strength hot-rolled steel sheet, superior in any of workability, fatigue property, and surface quality.
A demand for improvement in crash safety, and fuel economy of an automobile has recently become increasingly severer, and reduction in weight of an automobile body is highly desired as a countermeasure for meeting the demand. Since the weights of wheels and suspension parts, in particular, among automobile parts, occupy a high ratio in the weight of the automobile body in whole, the reduction in weight can be implemented if those parts are reduced in thickness by increasing the strength of a constituent material used in those parts.
Now, the wheels and suspension parts are worked on primarily by press forming, so that the constituent material thereof is desired to be superior in workability. Further, the constituent material need to be high in fatigue strength, and superior in durability so as to be able to minimize damage incurring while in use. In addition, even beauty of the external surface of some parts, as in the case of, for example, the wheels, is sometimes required.
Accordingly, there has been a demand for a high-strength hot-rolled steel sheet superior in any of properties such as workability, fatigue property, and surface quality, and as a method of producing the same, there has been proposed a technology disclosed in, for example, JP-A No. 9-31534. With the technology described, the surface quality as well as chemical processability of a steel sheet is improved by lowering Si content, and a ferrite phase is reinforced by adding Nb in combination with Ti to thereby enhance strength while improving workability. With the technology described, however, because Ti in combination with Nb are added as essential elements, a cost becomes high.